The invention relates to an apparatus and method for monitoring and measuring the relative amplitude of a predetermined number of electrical input signals and, more particularly, provides an apparatus and method that is useful in measuring, on a real time basis, the position of a beam of charged particles relative to the walls of a particle accelerator ring evacuated chamber or related device within which the beam is confined.
It is well known in the design and operation of particle beam accelerators, such as synchrotrons and storage rings, to provide beam position monitoring and measuring apparatus that is used to develop signals which are dependent upon the position of the beam relative to selected sections of the walls of the accelerator. Present day processes for measuring the position of a charged particle beam relative to the walls of an accelerator vacuum chamber involve placing a number of detector electrodes at selected points along the vacuum chamber walls to pick up signals induced in the electrodes by movement of the charged particle beam past the electrodes. Such measurements require the use of signals from a number of electrodes. Individual amplifier-detector equipment is normally required for each electrode involved in the measurement, and each such amplifier-detector must perform in precisely identical fashion, over a wide variation of signal strengths.
In addition, a means must be provided to normalize the results for variations in the magnitude of electrical charge on the beam of particles. This either involves the use of an electronic analog dividing circuit, which is expensive and difficult to adjust and has a limited dynamic range, or involves devices which must convert analog signals to digital signals, along with related digital processing equipment; both of which alternatives are cumbersome and expensive.
The position of a particle beam relative to its associated vacuum chamber walls is often influenced by mechanical motion of various elements of the accelerator, or by magnetic or electrical disturbances of an extraneous nature. It is necessary that the response time of the apparatus does not limit the ability of various bending-correction magnets to appropriately correct the changes in the beam position caused by these kinds of disturbances.
This invention resolves the problems outlined above. In particular, the invention (1) eliminates the need for a multiplicity of precisely matched amplifier and detector channels, in order to measure a plurality of beam position-dependent signals; and (2) eliminates the need for an analog or digital dividing circuit in such measurement applications. Furthermore, the response of the apparatus of the invention is sufficiently fast that its output can rapidly control charged particle beam bending magnets, thereby to make real-time corrections to the beam position, as required by the electrical disturbances and other accelerator perturbance factors previously mentioned.
In the preferred embodiment of the circuit of the invention, the detector and other circuit components following the amplifier need operate only over a limited linear range, which is independent of beam intensity.
One common method of detecting the position of a charged particle beam within an accelerator vacuum chamber is to mount a set of four pick-up electrodes at predetermined points on the chamber such that two of the pick-up electrodes are located above and two below the beamline. As charged particles in the beamline move past the pick-up electrodes, the capacitive coupling of the charged particles with the electrodes develops signals on the electrodes that are dependent on the position of the beamline relative to the electrodes. From such a set of four electrodes installed in an accelerator vacuum chamber, the transverse position of the charged beamline can be determined, using well known formulas for calculating certain sum and difference ratios between the radio frequency signals induced on the electrodes by their coupling with the beamline.